1. Field of the Invention
The present invention relates to a developing apparatus used for an image forming apparatus, such as a printer, a copier or a facsimile machine, and particularly to a developing apparatus that employs a plurality of developing devices mounted on a rotary member to perform multi-color developing for two or more colors.
2. Description of the Related Art
Recently, there has been a rapid increase in strongly expressed requests for the downsizing of color copiers (color image forming apparatuses) and improvements in the quality of images.
In order to respond to these requests, many image forming apparatuses of a single-drum, rotary-member developing type have been proposed, wherein single photosensitive drums are employed as image bearing members and a plurality of developing devices are mounted on rotary members (rotary developing devices), and wherein, for developing performance, the rotary members are rotated to move desired developing devices to developing positions opposite to the photosensitive drums.
These image forming apparatuses are provided to save space by mounting a plurality of developing devices on a rotary member, and to attain high image quality, through the stabilization of color hues, by providing a constant developing position on a photosensitive drum and by reducing the misregistration that can result from the use of a single-drum system.
Disclosed in the prior art is JP-A-Hei 09-288419 (see FIG. 9), which is a configuration, wherein not only developing devices 4a, 4b and 4c but also a toner supply unit 201 and toner cartridges 202a, 202b, 202c and 202d are mounted on a rotary member 4A, and all the components are rotated together. The features of this configuration are that a toner supplement path from the toner cartridges to the developing devices is short and requires only a simple structure, and that because of these features, the configuration can be provided at a low cost.
Disclosed in the prior art is JP-A-2001-134045 (see FIG. 10), which is a configuration wherein a toner supply unit 203 and a toner cartridge 204 are located outside a rotary member 206 on which a developing device 205 is mounted, and toner is supplied externally to the developing device 205 on the rotary member 206. The features of this configuration are that, since the toner cartridge 204 is not present inside the rotary member 206, the downsizing of the rotary member 206 is enabled, and that the capacity of the toner cartridge 204 can be increased.
Furthermore, image forming apparatuses have recently been proposed wherein added values, such as high speeds, high image quality and high functions, are also provided for the structures of a rotary member, or a rotary member and a developing device, and a toner cartridge.
For example, disclosed in the prior art is JP-A-2003-050494 (see FIGS. 11A, 11B, and 11C), which discloses a method for driving the rotary member 4A under a condition wherein developing devices are unevenly arranged. Furthermore, disclosed in the prior art is JP-A-2003-233239 (see FIG. 12), which is a method whereby developing devices 4a, 4b, 4c and 4d are arranged on one side of a rotary member 4, so as to easily drive the rotary member 4 when the developing devices are switched.
As is described above, the main purposes of the single-drum configuration, wherein the developing devices are mounted on the rotary member, are an improvement in the image quality and an increase in the space saved.
However, with the configuration wherein images are formed by rotating and changing the developing devices mounted on the rotary member, image smearing will occur due to the vibration, or the impact, that occurs when the developing devices are switched or the rotation is halted, so that this is an unfavorable configuration for improving the image quality, which is the original object.
Therefore, in the present situation, for many image forming apparatuses, various and numerous technical countermeasures are taken to switch between the developing devices, i.e., for the control and the structure of the rotary member.
Among these technical countermeasures, very effective ones are a technique for obtaining a balance in the rotation of the rotary member, a technique for reducing the size of the rotary member, so as to minimize the rotational inertia and rotational energy, and a technique for the moderate driving of the rotary member.
For the reduction in the size of the rotary member, balancing this with the downsizing of the developing devices must be considered in order to maintain the image quality.
When the technical theme for coping with the high image quality is considered, the technique proposed in JP-A-Hei 09-288419 has the following problems. Since the toner supply unit and the toner cartridges are mounted on and rotated with the rotary member and with the developing devices, it can be easily understood that the weight balance between the toner container and the developing devices on the rotary member will change as the toner is consumed. For this configuration, it is very difficult, for the rotational balance of the rotary member to be maintained. Further, when rotational vibration occurs, due to degradation of the rotational balance, or when there is an increase in the impact, due to the halting of the rotary member, it has a direct, adverse affect on the quality of an image, and rotational vibration is transmitted to an optical system and affects the scanning process, so that image deterioration occurs.
The method, disclosed in JP-A-2003-050494, for driving the rotary member while the developing devices are unevenly arranged has the following problems. The method used to reduce the speed of the rotary member, when the balance is inappropriate, insufficiently reduces the uneven rotation and the shock that occurs when the rotary member is inappropriately balanced. Since in this case the speed of the rotary member is reduced, this is an effective configuration that can provide control for the output of a drive motor and for a torque margin; however, there is a considerable reduction in the productivity of the image forming apparatus.
With this configuration, the developing devices are switched at the interval between recording media, and it is highly probable that the number of image forming faces will be restricted and a reduction in the productivity cannot be avoided.
According to the configuration described in JP-A-2003-233239, wherein the developing devices are arranged along one side of the rotary member, the rotary member for changing the developing device can be driven easily because only a small rotational angle is required. However, it can easily be understood that, even when toner cartridges are provided to maintain the balance, the balance of the rotary member is lost as the toner is consumed. Thus, the deterioration of an image due to the poor balance of the rotary member can not be avoided. Further, with this configuration, since the developing devices are located along one side of the rotary member, the diameter of the rotary member is increased. As a result, the rotational inertia of the rotary member is increased, and accordingly, the impact when the rotary member is halted is increased, so that image deterioration occurs. To resolve this problem, the developing devices must be thin and have reduced sizes, or else stable circulation of the developer and developing performance will be adversely affected. Otherwise, image forming will be performed only at a speed consonant with the size of the developing devices, and the productivity will be reduced.
While taking the above description into account, the configuration as proposed in JP-A-2001-134045 is preferable in order to drive the rotary member without uneven rotation being caused. That is, it is preferable that the toner supply unit and the toner cartridges be located separately from the rotary member, whereon the developing devices are mounted, and supply toner to the developing devices.
According to this arrangement, since the toner cartridges are located separately from the rotary member, a change in the weight balance as the toner is consumed does not affect the rotary member, and by controlling the toner density, a constant amount of toner can be maintained in the developing devices on the rotary member, regardless of the toner color.